1. Field of the Invention
The present invention relates to an idling mode detecting switch for an internal-combustion engine.
2. Description of the Prior Art
In an internal-combustion engine, fuel supply rate is controlled according to the degree of opening of the throttle valve, and an idling mode detecting switch is used for detecting the idling mode of the internal combustion engine, in which the throttle valve is in an idling position.
Referring to FIG. 1 showing a conventional idling mode detecting switch, a substantially tubular housing 2 attached to a throttle body 1 is internally provided with a push rod 3, a movable contact 4 provided at the lower end of the push rod, and a fixed contact 5 disposed opposite to the movable contact 4. The throttle body 1 is mounted on an engine, not shown, and is grounded through the engine. The push rod 3 is slidably supported in a diametrically small guide portion 6a of a substantially tubular guide sleeve 6. The upper end of the push rod 3 projects from the upper opening of the housing 2. An annular plate 7 is mounted coaxially on the lower portion of the push rod 3 extending within a diametrically expanded portion 6b of the guide sleeve 6. A substantially tubular insulator 8 having a flange 8a is disposed within the lower expanded portion of the housing 2 with the flange 8a thereof fitting the opening of the lower expanded portion of the housing 2. The lower end of the push rod 3 is received slidably and close in the boss 8b of the insulator 8. A lead plate, 9 is provided between the lower end surface of the diametrically expanded portion 6b of the guide sleeve 6 and the flange 8a of the insulator 8. A spring 10, i.e., a resilient member, is extended between the plate 7 and the lead plate 9 so as to bias the push rod 3 toward an actuating end portion 12a of a stopper 12 connected to the valve shaft 11 of the throttle valve. When the stopper 12 is brought into contact with the upper end of the push rod 3 to apply pressure thereto, the push rod 3 is caused to slide axially downward against the resilience of the spring 10.
The fixed contact 5 is connected to a terminal 13 extending outside through a resin connector 14 fitted in the lower opening of the housing 2. The connector 14 is secured to the housing 2 by caulking a fastening portion 15 of the lower extension of the housing 2. The connector 14 serves also as an insulating cap for closing the lower opening of the housing 2. The outer circumference of the upper portion of the housing 2 is threaded to form a threaded portion 2a, which is screwed in a threaded hole 1a formed in the throttle body 1 to attach the housing 2 to the throttle body 1. Indicated at numeral 16 is a sealing rubber boot put on the upper end of the housing 2.
The function of the idling mode detecting switch thus constructed will be described hereinafter.
The valve shaft 11 of the throttle valve turns clockwise, as viewed in FIG. 1, as the degree of opening of the throttle valve is decreased. The stopper 12 is positioned relative to the valve shaft 11 so that the actuating end portion 12a of the stopper 12 presses the push rod 3 downward against the resilience of the spring 10 to bring the movable contact 4 provided on the lower end of the push rod 3 into contact with the fixed contact 5 at a particular degree of opening of the throttle valve. Then, an electric current supplied through the terminal 13 flows through the fixed contact 5, the push rod 3, the plate 7, the spring 10, the lead plate 9, the housing 2 and the throttle body 1 to the earth, and thereby the idling mode of the internal-combustion engine is detected.
In this conventional idling mode detecting switch, the plate 7 and the lead plate 9 are kept in contact with the opposite ends, respectively, of the spring 10 for electrically connecting the spring plate 7 and the lead plate 9 merely by the resilience of the spring 10. In such a case, it is possible that a detection signal of a sufficiently high level is unavailable due to contact resistances between the plate 7 and the spring 10 and between the lead plate 9 and the spring 10. Such an adverse tendency is enhanced when the contact surfaces of the plate 7 and the lead plate 9 are smeared with oil or oxidized. The electrical contact between the plate 7 and the spring 10 and between the lead plate 9 and the spring 10 is unstable and the level of the detection signal is liable to change for the same reason because the condition of contact between the plate 7 and the spring 10 and between the lead plate 9 and the spring 10 changes if the spring 10 turns relative to the plate 7 and the lead plate 9.
Furthermore, every time the actuating end portion 12a of the stopper 12 attached to the valve shaft 11 of the throttle valve is brought into contact with the push rod 3, the action pressure applied to the push rod 3 by the stopper 12 is transmitted through the fixed contact 5 directly to the connector 14 serving also as the insulating cap to expand and loosen the caulked fastening portion 15. Consequently, the connector 14 is moved outward relative to the housing 2 causing the idling mode detecting position to deviate from the correct position, or a portion of the connector 14 adjacent to the caulked fastening portion 15 is caused to creep.